Alfie is now 5.5 months old. He is crated at night in our bedroom in a really large crate and he sleeps very well and through the night.

If we leave the house he is crated in a smaller crate downstairs and left nolonger than 3-4 hours ( usually only 2hrs ) again he sleeps well and doesnt soil the crate.

He will ask now to go outside to soil, usually by just sitting at the patio door. He doesnt pee in the house except on the odd occassion we havent watched him properly and this is usually near to the patio door.

So basically he is very good and always asks but the problem is when we are in the house he asks to go out ALOT. Probably once every half hour or so. This isnt a problem, as he does ask, but i just dont know how we will progress to the next stage of housetraining i.e. leaving him out of the crate whilst we leave the house. How do we teach him to hold his bladder out of the crate??

do what i do, invest in a dog flap. once hes older and is loose in the house he can go in/out as required

My dogs have been crated untill about a year, mainly because it is our routine and they are happy and secure and it minimizes chewing risk.
By that time they know that outside is the place to go and can  and do wait. My little pom/chi will probably always be crated for her safety as I dont like leaving them unsupervised because of their size difference
You seem to be doing fine I would carry on as you are crating when you go out and at night he sounds like he is doing great.
Personally I dont like the idea of dog flaps (security) but I do have a cat flap which the pomXchi could use if she wanted but doesnt but I would need a large one as I have a gsd too ! I am not sure what breed you have.
  

If you wanna go out and leave him uncrated then do lil steps..

Don't make a big deal you are leaving- but always make sure you put him out before you do leave. Then do lil ones, one time leave for 10 min, next time 5, another 15- always do different times so he doesn't no when to expect you home. Don't fuss him up alot when you get home cause that to can cause him to pee from excitement. Then straight out when you get home. This is what I did with my dogs and I have 4 that run lose all day have been since 5-6 months. Mine always have their crate doors open and can come and go at will. Make sure there is nothing he can get into while you are out like wires, garbage ect. You can also give him a stuffed kong only for when you go out and he will also look forward to that as well and keep him busy.

do what i do, invest in a dog flap. once hes older and is loose in the house he can go in/out as required

A dog flap is the best thing i did. This way they can go in and out when they need whether you're home or not. As for security, if burglars want to get in, they'll get in..

I know some people leave their door open all day, but i imagine a lot of creepy crawlies/rodents will come in too, i live in Australia and my house would be over run with flies and mosquitoes to boot

Dog thefts from runs and gardens are running at around ten a week in the UK. There is no way that I could go out without making sure that my dogs were securely contained.

Jeepers CarolC, that's terrible! I haven't heard of dog thefts happening here, but I'm sure it is not unheard of.

> Dog thefts from runs and gardens are running at around ten a week in the UK. There is no way that I could go out without making sure that my dogs were securely contained.

Absolutely!   And then there's the possibility of them making a racket outside when I'm not home, bringing neighbours to my door.

> There is no way that I could go out without making sure that my dogs were securely contained.

There is also the issue of neighbour nuisance a dog free to go in garden is likely to bark at anything of interest, with no-one to tell them not to.

And they do, as mine proved when I had one.  Not something I would risk again - at the time I thought my neighbours were very nice but when their true colours began to show, I often wonder if they did anything when I wasn't there as one of mine was worried about them.